1. Field
This invention relates to the field of aircraft such as civil transport aircraft.
More particularly, the invention relates to an aircraft that has at least one vertical fin fastened to the fuselage in a rear area of the fuselage, the geometric configuration of which is modified to reduce drag while cruising.
2. Brief Description of Related Developments
Modern transport aircraft, in particular civil passenger transport aircraft, at least for the most widely used aerodynamic architectures, have a group of fins fastened at the rear of a main aircraft structure that itself includes a vertical fin, most often fastened to the fuselage and above it, essentially in a vertical plane of symmetry of the aircraft.
The vertical fin has a dual role.
On the one hand, by virtue of the aerodynamic forces that it generates on the aircraft, the vertical fin assures a natural directional stability of the aircraft.
This directional stability is due to the restoring torque Cn around a vertical axis Z of a conventional aircraft configuration when the aircraft is subjected to an aerodynamic sideslip β, in other words the aerodynamic coefficient Cnβ=dCnβ/dβ is negative and tends to cancel the sideslip. The torque Cn originates from a lateral lift along Y of the vertical fin when the aerodynamic sideslip β is not zero.
On the other hand, by means of a directional rudder, in other words an aerodynamic surface hinged to the trailing edge of the vertical fin, the vertical fin is able to generate a yawing torque that is a function of the setting of the directional rudder, proportional to a first approximation to the angle of the rudder setting.
This rudder is used to direct the aircraft when the flight conditions are not symmetrical on the aerodynamic level.
Such asymmetric flight situations are encountered when the aircraft is subjected to lateral aerodynamic flow, for example in the presence of gusts of wind that have a horizontal component and especially when the aircraft is in a dissymmetric propulsive engine thrust configuration, essentially in case of an engine breakdown.
In these asymmetric configurations, a more or less durable action on the directional rudder is necessary to assure the control of the aircraft in flight or on the ground, and the efficacy of the control depends on the dimensions in particular of the reference aerodynamic surfaces of the vertical fin and of its rudder, as well as on the geometry of the fin following the known effects of parameters such as elasticity, deflection, or others.
In practice, the aerodynamic forces generated by the vertical fin are directly proportional to the dynamic pressure Pd of the aerodynamic flow, in other words it is a function of the square of the airspeed of the aircraft: Pd=½ ρV2, ρ being the density of the air at the point of flight under consideration, and V being the relative speed between the aircraft and the flow.
In practice, the dimensions of the vertical fin, both of its fixed part and of its rudder, are most often established for the most unfavorable flight conditions, in other words at low speed when the dynamic pressure Pd is low and when the dissymmetry of flight is at a maximum, which occurs in general when the thrust of the engines is maximal with failure of an engine off center relative to the plane of symmetry of the aircraft, with the ability of the aircraft to control its trajectory then defining a minimum control speed VMC,
The summation of these two condition (low dynamic pressure and dissymmetry) is encountered effectively in case of an engine failure during a low-speed phase of flight, at takeoff or when restoring gas on approach, in flight or on the ground during an acceleration, when the speed no longer assures the lateral control of the aircraft by acting on the wheels, and taking this critical condition into account imposes a vertical fin of large dimensions even though when the speed is higher than during the takeoff or approach phases, a vertical fin of relatively smaller dimensions would be sufficient to assure correct directional stability.
The vertical fins of current aircraft accordingly have dimensions determined for critical conditions that concern only a limited period of the flight of an aircraft, and have dimensions greater than those needed for the longer period of flight, which is penalized in particular considering a traversed surface of no use in the aerodynamic flow, that generates aerodynamic drag.